This invention pertains to apparatus for producing uniform, high luminance light and, more particularly, to a system for producing uniform, high luminance light in a large area, back light system for flat panel displays.
Large flat-panel displays made in accordance with known active matrix (or TFT) liquid crystal display technologies are typically mounted in front of a back light module which L contains an array of fluorescent lamps. FPDs of this type have been increasing in size annually by about 1 to 2 inches, diagonally. The median size in 1999 for use in desktop PCs was about 15 inches diagonal view area. A few very large displays are made in the range of 20 to 25 inches diagonal. Tiled AMLCD FPDs may be made in the range of 40 inches diagonal, as described in copending U.S. patent application Ser. No. 09/368,921, assigned to the common assignee and hereby incorporated by reference.
However, tiling, as described in U.S. Pat. No. 5,661,531, and also included by reference, requires extremely intense light sources with substantially collimated lighting, masked optical stacks, and pixel apertures that have very low emitted light efficiency. Thus, lighting with unusually high intensity ranges of 50,000 to 150,000 nits is desirable with uniformity over very large FPD areas. Unique designs and control features are necessary to achieve such high intensities at reasonable wattages for consumer or business applications.
Maintaining such a bright illumination uniformly over the entire active area of the display is difficult to do. The intensity required for some applications, and in particular, that required for a large tiled flat panel LCD display as described in U.S. Pat. No. 5,867,236, issued Feb. 2, 1999, entitled CONSTRUCTION AND SEALING OF TILED, FLAT-PANEL DISPLAYS, causes the lamps to produce a significant amount of heat. Moreover, fluorescent lamps are designed to run most efficiently at an elevated temperature, so it is desirable to operate them at a predetermined ideal design temperature, which is usually in the range of 50 to 60 degrees Centigrade.
Small, edge-lit, back light modules used in notebook or laptop PCs do not produce sufficient brightness for a large area display, nor are they capable of illuminating a large area uniformly. Thus it is necessary to illuminate the area with an array of fluorescent lamps. The number of lamps required depends on the size of the area to be illuminated and the display brightness specifications. A large area display needs multiple lamps to illuminate it properly.
Since most displays are designed to be wider than they are tall, it is advantageous from a reliability and power perspective to use horizontal lamps. This results in fewer lamps and less power, since fewer lamp cathodes are required. The resultant designs use lamp tubes placed horizontally, one above the other. This produces a chimney effect, the upper lamps receiving heated air from the lamps below. As expected, the temperature differential from top to bottom can become severe. Unfortunately, lamp tube temperature differences cause significant variations in the luminance of the back light and contribute to decreased life expectancy.
Fluorescent lamps, particularly high efficiency hot cathode types, operate with a significant amount of the power consumption at the ends (cathodes). This naturally produces high temperatures at the cathodes of the lamp tube. A typical lamp operates in open air with a tube wall temperature preferably at about 55 degrees Centigrade, while the end may be higher than 85 degrees.
This invention provides a unique conduction cooling structure means for uniformly distributing the heat generated by the lamp tube cathodes, thus helping to maintain maximum brightness by keeping all of the lamp tube ends at or very near a uniform temperature. The temperature of the lamp ends is kept near the temperature of the central section of the lamp tube, preferably about 55xc2x0 C., which provides for uniform brightness along the lamp tube within a few percent at peak efficiencies and ensures the longest possible lamp life.
This invention further provides unique means for directing cool fresh air to impinge on predetermined portions of lamp tubes so as to develop cooling means and uniform temperature distributions in the stack of bulbs. The invention is also capable of providing a more uniform temperature distribution across the array of lamp tubes in a high luminance output back light module for a large area flat panel display.
Additionally, when used in combination with the invention disclosed in copending U.S. patent application Ser. No. 09/407,619 (RDI-125), filed Sep. 28, 1999, hereby incorporated by reference, the present invention provides a very uniform, high luminance back light system capable of maintaining brightness within a few percent over periods of days under a wide range of environments. It is particularly suited for the application of a back light system for a large tiled, flat panel LCD. Such an application is disclosed in copending U.S. patent applications, Ser. No. 09/409,620 (RDI-127), filed Sep. 28, 1999 and Ser. No. 09/368,291, filed Aug. 6, 1999, both also incorporated herein by reference.
In accordance with the present invention, there is provided apparatus for uniformly distributing luminance from a back light module for a flat panel, liquid crystal display (LCD). Fluorescent lamps are commonly used in back light modules for LCDs due to their high efficiency. Luminance from fluorescent lamps is a function of lamp tube temperature, as is the efficacy of the lamp and the operating life thereof. This invention provides means for achieving luminance uniformity, high efficiency and long life by distributing the lamp cathode thermal energy and maintaining uniform lamp wall temperatures.
A unique heat sink attachment conduction cools the cathode areas of the fluorescent lamps. Cooler operating temperatures are achieved at the lamp ends, which has two significant benefits. First, the lower operating temperature of the cathode increases the lamp life, and second, provides for more even distribution of temperature and, therefore, uniform lamp luminance output in the range of a few percent over the length of the tube. A thermal sensor is also mounted in the heat sink body. In addition, open louver slots positioned behind the lamps allow for cool air to enter behind each lamp. The size, shape and position of these louvers can be selected so that the lamp temperatures are essentially constant over their entire length.
A constant and uniform luminance output of the back light module is further obtained through appropriate selection of lamps, reflective back light cavity and light diffuser. This invention provides means for achieving very high and uniform luminance output, 35,000 to 150,000 nits, over a very large surface area at minimal power consumption through appropriate design of the cathode heat sinks in conjunction with a set of specific air inlet louvers.
The cathode heat sinks also provide an optimum location for locating a temperature sensor. The sensor can be used in a control system, such as that described in the aforementioned patent application, Ser. No. 09/407,619, to efficiently manage the back light output.